Reference-less clock and data recovery circuit

Patent metadata

Field	Value
Publication number	US-9793902-B2
Application number	US-201615048040-A
Country	US
Kind code	B2
Filing date	Feb 19, 2016
Priority date	Feb 19, 2016
Publication date	Oct 17, 2017
Grant date	Oct 17, 2017

Abstract

Embodiments herein describe a reference-less CDR circuit that receives electrical signals that may have been transmitted along either an electrical or optical interconnect which are then processed to identify the original data. To do so, the CDR circuit includes a frequency locking loop (FLL) and a phase locking loop (PLL) which generate control signals for a voltage controlled oscillator (VCO). In one embodiment, the FLL generates a coarse adjustment signal which the VCO uses to output a recovered clock that substantially matches the frequency of the received electrical signal. The PLL, on the other hand, generates a fine adjustment signal which the VCO uses to make small adjustments (e.g., half cycle phase shifts) to the recovered clock. The recovered clock outputted by the VCO is then fed back and used as an input in both the FLL and the PLL.

First claim

We claim:

1. A reference-less clock and data recovery (CDR) circuit, comprising:
a frequency locking loop (FLL) comprising a frequency detector configured to generate a first control signal based on a frequency difference between a received data signal and a recovered clock, wherein the FLL is configured to generate a second control signal to deactivate the FLL and activate a phase locking loop (PLL) upon determining the FLL is in a locked state, wherein the FLL comprises:
a compressor configured to compress a digital value represented by the first control signal and switch between two modes of operation based on a difference between the received data signal and the recovered clock as measured by the frequency detector,
a digital-to-analog converter (DAC) configured to output a current based on the compressed digital value, and
an analog integrator;

the PLL comprising a phase detector configured to generate a third control signal based on a phase difference between the received data signal and the recovered clock; and
an oscillator configured to adjust the recovered clock based on the first and third control signals, wherein the analog integrator is configured to receive the current and output a voltage for driving the oscillator in order to adjust the recovered clock.

2. The CDR circuit of claim 1 , wherein the PLL is inactive when the FLL is active and the FLL is inactive when the PLL is active.

3. The CDR circuit of claim 2 , wherein the frequency detector in the FLL generates the second control signal, and wherein the FLL further comprises selection logic which breaks a loop formed by the FLL in response to the second control signal indicating the FLL is in the locked state, and
wherein the PLL further comprises a switch activated by the second control signal, wherein the switch transmits a constant voltage to the oscillator when the PLL is deactivated.

4. The CDR circuit of claim 1 , wherein the FLL comprises:
a first frequency divider configured to divide the received data signal by a first predefined value; and
a second frequency divider configured to divide the recovered clock by a second predefined value,
wherein respective outputs of the first and second frequency dividers are inputs for the frequency detector.

5. A reference-less clock and data recovery (CDR) circuit, comprising:
a frequency locking loop (FLL) comprising a frequency detector configured to generate a first control signal based on a frequency difference between a received data signal and a recovered clock, wherein the FLL is configured to generate a second control signal to deactivate the FLL and activate a phase locking loop (PLL) upon determining the FLL is in a locked state, wherein the FLL comprises:
a compressor configured to compress a digital value represented by the first control signal,
a digital to analog converter (DAC) configured to output a current based on the compressed digital value, and
an analog integrator;

the PLL comprises a phase detector configured to generate a third control signal based on a phase difference between the received data signal and the recovered clock and a comparator configured to output an integrator adjustment signal to control an input of the DAC when the PLL is active and the FLL is inactive, wherein the integrator adjustment signal causes the DAC to at least one of increase and decrease charge across a capacitor in the analog integrator; and
an oscillator configured to adjust the recovered clock based on the first and third control signals, wherein the analog integrator is configured to receive the current and output a voltage for driving the oscillator in order to adjust the recovered clock.

6. The CDR circuit of claim 5 , wherein the integrator adjustment signal is based on comparing a value derived from the phase difference between the received data signal and the recovered clock to a predefined reference value.

7. A reference-less clock and data recovery (CDR) circuit, comprising:
a frequency locking loop (FLL) comprising a frequency detector configured to generate a first control signal based on a frequency difference between a received data signal and a recovered clock, wherein the FLL is configured to generate a second control signal to deactivate the FLL and activate a phase locking loop (PLL) upon determining the FLL is in a locked state;
the PLL comprises:
a phase detector configured to generate a third control signal based on a phase difference between the received data signal and the recovered clock,
a charge pump coupled to the phase detector to receive the third control signal, and
a loop filter coupled to the charge pump and configured to output a control voltage for adjusting an analog integrator in the FLL; and

an oscillator configured to adjust the recovered clock based on the first and third control signals, wherein the charge pump is configured to output a fine adjustment signal for controlling the oscillator.

8. An integrated circuit, comprising:
a frequency locking loop (FLL) comprising a frequency detector configured to generate a first control signal based on a frequency difference between a received data signal and a recovered clock, wherein the FLL is configured to generate a second control signal to deactivate the FLL and activate a phase locking loop (PLL) upon determining the FLL is in a locked state;
the PLL comprising:
a phase detector configured to generate a third control signal based on a phase difference between the received data signal and the recovered clock
a charge pump coupled to the phase detector to receive the third control signal, and
a loop filter coupled to the charge pump and configured to output a control voltage for adjusting an analog integrator in the FLL; and

an oscillator configured to adjust the recovered clock based on the first and third control signals, wherein the charge pump is configured to output a fine adjustment signal for controlling the oscillator.

9. The integrated circuit of claim 8 , wherein the PLL is inactive when the FLL is active and the FLL is inactive when the PLL is active.

10. The integrated circuit of claim 9 , wherein the frequency detector in the FLL generates the second control signal, and wherein the FLL further comprises selection logic which breaks a loop formed by the FLL in response to the second control signal indicating the FLL is in the locked state, and
wherein the PLL further comprises a switch activated by the second control signal, wherein the switch transmits a constant voltage to the oscillator when the PLL is deactivated.

11. The integrated circuit of claim 8 , wherein the FLL comprises:
a compressor configured to compress a digital value represented by the first control signal;
a DAC configured to output a current based on the compressed digital value; and
the analog integrator configured to receive the current and output a voltage for driving the oscillator in order to adjust the recovered clock.

12. The integrated circuit of claim 11 , wherein the compressor is configured to switch between two modes of operation based on a difference between the received data signal and the recovered clock as measured by the frequency detector.

13. The integrated circuit of claim 11 , wherein the PLL comprises:
a comparator configured to output an integrator adjustment signal to control an input of the DAC when the PLL is active and the FLL is inactive, wherein the integrator adjustment signal causes the DAC to at least one of increase and decrease charge across a capacitor in the analog integrator.

14. The integrated circuit of claim 13 , wherein the integrator adjustment signal is based on com

Assignees

• Cisco Tech Inc

Inventors

• Lakshmikumar Kadaba
• Tse Mark Y
• Das Bibhu
• Dama Bipin

Classifications

• H03L7/0891

  the up-down pulses controlling source and sink current generators, e.g. a charge pump · CPC title

• H03L7/091

  the phase or frequency detector using a sampling device (H03L7/087 takes precedence) · CPC title

• H04L7/0331

  with a digital phase-locked loop [PLL] processing binary samples, e.g. add/subtract logic for correction of receiver clock (H04L7/0337 takes precedence) · CPC title

• H03L7/0807 Primary

  concerning mainly a recovery circuit for the reference signal · CPC title

• H03L7/095

  using a lock detector (H03L7/087 takes precedence) · CPC title